1. Field of the Invention
The present invention relates to piezoelectric thin-film resonators including an aluminum nitride material and particularly relates to a piezoelectric thin-film resonator including an Sc-containing aluminum nitride layer.
2. Description of the Related Art
Various piezoelectric thin-film resonators including piezoelectric thin films have been proposed. Japanese Unexamined Patent Application Publication No. 2011-15148 discloses a method for producing a piezoelectric thin film for use in such a piezoelectric thin-film resonator. That is, Japanese Unexamined Patent Application Publication No. 2011-15148 discloses a technique in which aluminum and scandium are sputtered at a substrate temperature of about 5° C. to 450° C. in a nitrogen atmosphere. This allows the content of scandium in an aluminum nitride thin film containing scandium to be about 0.5 atomic percent to 50 atomic percent, leading to an increase in piezoelectric response. Japanese Unexamined Patent Application Publication No. 2011-15148 describes that the content of scandium in the scandium-containing aluminum nitride thin film is about 0.5 atomic percent to 50 atomic percent as described above, preferably about 35 atomic percent to 43 atomic percent, and most preferably about 43 atomic percent.
However, in the case of fabricating a piezoelectric thin-film resonator using an Sc-containing aluminum nitride thin film having an Sc content described in Japanese Unexamined Patent Application Publication No. 2011-15148, there is a problem in that the absolute value of the temperature coefficient of frequency TCF of the piezoelectric thin film is large. Therefore, it has not been possible to obtain a piezoelectric thin-film device which has a sufficient fractional bandwidth and in which the absolute value of the temperature coefficient of frequency TCF is small.
In a piezoelectric thin-film resonator including the Sc-containing aluminum nitride thin film disclosed in Japanese Unexamined Patent Application Publication No. 2011-15148, properties according to the concentration of Sc can be obtained; however, it is difficult to vary the properties thereof. That is, the degree of freedom in designing various properties such as fractional bandwidth and frequency-temperature properties is low.